The history of telecommunications includes several periods, the first of which began in the Stone Age when people learned to speak. It was followed by postal services, pigeon service, telegraph service, telephone and radio communications, and by the end of the 20th century there appeared another one ??“ cellular communications. It is not necessary to describe the differences between cellular communications and other means of communications as long as the average reader has already got an idea about the space propagation of radio waves and is likely to experience their influence feeling the vibration of a mobile phone in the pocket.
A mother of children who are playing in a sandpit wouldn??™t cry for them from the balcony but would rather call them using her cellular phone. Business partners are always available for each other. A young man sitting in a park with his notebook and cellular phone wouldn??™t carry thousands kilometers of wires in order to connect to an Internet provider. All this has become possible when using a mobile cellular communications, the creation, evolution and the future of which will be discussed further.
The reason for creation and evolution of cellular communication was the necessity for communications to become countrywide and affordable. The first attempt to implement mobile telephony was made in 1946 in St.Lewis (USA) when a simple 6-channel transceiver was created. Only 24 years later scientists started the development of the ???wireless??? telephony again. The architecture and performance principles of cellular network were clearly described in 1975. In the result of almost 10-years work there appeared NMT-standard (Nordic Mobile Telephone), which was intended to work in a 453.0-457.5 MHz band and had 180 channels 25 KHz each. In 1978 NMT was introduced as a standard by Denmark, Finland, Iceland, Norway and Switzerland.
A modification of NMT-900, which performed at 900 MHz and had some advantages in comparison with the first standard NMT-450, emerged a bit later. In particular, there appeared an opportunity to produce portable mobile phones and also the quality of communications increased.
By the beginning of the 80s there were telephones constructed, which let to make calls to another phones with the aid of base stations. These devices pertain to the first generation of cellular communications, or 1G.
One of the first of the NMT-450-standard telephones by Nokia
Mobile telephones of the first generation were no less than a size of a suitcase and included a base and a separate handset, which was impossible to carry. Nowadays it is hard to imagine such a wizardry, which weighed several kilos, emanated 20-30 watt signal and had a 30-40 cm antennae. The maximal voice bitrate was 9.6 Kb/s, and data rate was 1.9 Kb/s. On a European market the NMT-standard was introduced by somewhat different standards: AMPS (Advanced Mobile Phones Service) and TACS (Total Access Communications System). The disadvantages of analogue standard NMT are high level of radiation, interference sensitivity and a low level of privacy of the transmitted data.
2G standard was introduced to solve the problems of cellular communications of the first generation, the development of which began long before the public implementation of NMT-networks.
In 1982 on the European Conference of Postal and Telecommunications Administrations there was formed Groupe Speciale Mobile, or GSM, the aim of which was to develop a reliable and simple system of terrestrial mobile communications. The system had to provide high quality of the transmitted voice data, simple construction and maintenance of cellular network, support of working with portable devices, such as PDA, notebooks etc., possibility to realize not only local, but international roaming, openness for the development of new services, compatibility with ISDN systems and effective air usage.
The right to proceed with the development of the standard was imposed on ETSI (European Telecommunication Standards Institute). In the middle of 1990 there were introduced specifications of standard GSM-900, which then meant Global System for Mobile communications. Then there was evolutional standard GSM-1800, which used radio frequency 1800 MHz. The increase of the radio signal frequency two times let to increase density of subscriber base four times.
GSM-900 applies restrictions on the remoteness of mobile phone from the base station, which cannot exceed 35 km even if the signal power is enough. This is because packets with overcome life-span are culled. In GSM-1800 the coverage is even less: about 6-10 km. NMT doesn??™t have such restrictions and in perfect conditions it is possible to realize communications at 200 km (experimentally proved). It is needed to say that reducing coverage two times lets to increase density of subscriber base two times. There is a special configuration of base stations, Extended Cell, where the distance may be increased up to 70-100 km, but the amount of simultaneously serviced subscribers cannot exceed 2-3
The uniqueness of the new GSM standard is that it is the first one to use digital cellular system as distinct from the existing analogue systems. Its practical usage was started in 1991. The data bitrate in the network increased from 1.9 Kb/s to 14 Kb/s. That let to use mobile phone, which by that time could be named that, modem or fax, and afterwards to use WAP-services.
Primarily GSM was very flexible in the amount and diversity of services. Due to the new standard there appeared Short Message Service, or SMS. This service let to exchange short text messages up to 160 symbols. Based on SMS protocol there broadcasting appeared ??“ news and other information was broadcasted to all the subscribers of the network. An additional flexibility was achieved by using SIM-cards (Subscriber Identity Module), which let to assign to the net not the telephone itself but a small module, which included international user??™s identifier of mobile services.
TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) and PDC (personal Digital Cellular) have had a wide distribution apart from GSM.
In July of 1989 TDMA was accepted by a range of large mobile companies in the USA as a replacement of the existing analogue standards. That can be counted as the first commercial maintenance of such network.
TDMA is a technology of transmitting digital signal. TDMA lets to avoid interference and lets to use radio signal most effectively. The principle of operation is as follows: when calling, a telephone is given a frequency channel, which is divided into equal timeslots. When a telephone transmits an encoded audio signal over a channel, all the other telephones are waiting for their turn. In the next timeslot another telephone transmits and so on. Timeslots have such length, that lets to share the air imperceptibly for human ear. The advantages of TDMA:
- the most effective, simplest and recompensed standard for the transfer from analogue transmission of AMPS signal to digital;
- data transmission from 64 Kb/s up to 120 Mb/s;
- no interference from other telephones;
- prolonged time for working from accumulators, which is because the signals are transmitted periodically, in contradiction to 1G networks, where signal is transmitted all the time;
- high network capacity.
Along with the advantages there are disadvantages of the technology:
- impossible in some cases to continue talk in the time of switching between base stations;
- possibility of interference from buildings because of the simultaneous transmission of the signal to several available base stations.
TDMA is compatible with many different cellular networks standards, and its principles are used in several third-generation networks technologies. Its flexibility, reliability, scalability influenced on the distribution of the standard, which is partially used in European GSM and 3G and 3.5G technologies.
CDMA is thought to be an obvious competitor of TDMA. The principle, which is used in signal transmission in such networks, pertains to military technologies and is used in military satellites. CDMA networks appeared in the USA a few months after TDMA, and they still exist and are constantly modernized. The increased network capacity and easiness of creation was the reason for quick spreading of the standard, and by the year 2003 the amount of its users was more than 150000000.
The principle of CDMA performance lets to avoid interference, to increase the number of simultaneously talking subscribers and to protect the transmitted information. During a call the encoded information is simultaneously transmitted from a telephone to several available base stations, which lets to choose the most ???rightful??? received packet. Apart from that, the transmitted signal is spread in frequency-time space, in contradiction with other radio technologies, where all the signal power is concentrated on the chosen frequencies or time intervals. In fact, only noise will be heard in the air, from which it is very hard to select wanted signal. That is why the technology has a very high protection level from overhearing. The data transmission bitrate varies from 9.6 Kb/s to 1.23 Mb/s.
CDMA technology has almost no disadvantages, provides high reliability of the voice and data transmission, required data protection level and sutureless switching between base stations. The principles of CDMA networks are widely used in third generation networks.
The coming of the second generation networks to market and a rising necessity of the mobile access to Internet were the reason for development of cellular communications in the direction of a bitrate increase. The development of 2.5G standards, which are, in fact, a buildup for second generation networks, started in 1985. The 2.5G standard in known to us as GPRS (General Packet Radio System), EDGE (Enhanced Data for Global Evolution), 1xRTT (2.5G CDMA data service). The maximum data transmission bitrate, which is possible to achieve using these standards, is 384 Kb/s.
GPRS technology lets to simultaneously talk on a telephone and to transmit data. It has become possible because of a more effective use of radio band and new coding methods. One more feature of the service is that it??™s not necessary to call to a provider every time. Telephone is always online, but it transmits data only when there is such need. This working scheme lets to simultaneously serve much more clients than when using second generation GSM networks.
As any other system, GPRS has its own disadvantages:
- Maximum theoretical speed, which is possible with GPRS-connection, is 172.2 Kb/s. The real bitrate rarely exceeds 48-50 Kb/s, which is because of redundancy of the transmitted data that provides protection from errors and includes control information;
- GPRS uses ineffective modulation technology GMSK (Gaussian minimum-shift keying). A more modern service EDGE uses a better technology 8 PSK (eight-phase-shift keying), that lets to achieve higher bitrates of data transmission..
The main advantage of 2.5G is a possibility to develop and apply completely new net services, the performance of which would be simply impossible in previous generation of standards. The average bitrate of 48 Kb/s for GPRS is enough for communication using IRC or IM-client, which, in practice, is much more convenient than interchanging SMS, internet-surfing using web-browsers and even for online video- or radio-broadcasting. But the main thing which 2.5G networks let to accomplish was the beginning of an integration process of Internet and cellular networks.